Project Summary/Abstract (Core B: HS Breeding Core) The N/NIH heterogeneous stock (HS) breeding core, under the direction of Dr. Leah Solberg Woods at the Wake Forest University School of Medicine (WFUSM), will provide HS rats to Research Project 1-3. HS animals are outbred from eight inbred rat strains and maintained in a manner that minimizes inbreeding, enabling the fine-mapping of quantitative trait loci (QTL). Dr. Solberg Woods? laboratory has maintained the HS rat colony since 2006 and hers is the only laboratory in the North America to house this unique resource. Over the past four years as part of the P50 Center for Excellence HS Breeding Core, the HS rat colony has become a national resource. We have sent rats to investigators both inside and outside the P50. Pilot and Feasibility grants funded through the P50 have led to funding for four NIDA-funded U01 grants that use HS rats. The two main goals of this core will be to 1) maintain the HS rat colony in a way that minimizes inbreeding and 2) ship a genetically diverse set of HS rats to Research Projects 1-3. To minimize inbreeding and control genetic drift, as well as to have sufficient rat to send to investigators within the P50 as well as currently funded U01 grants, the HS rat colony will increase from 64 to 92 breeder pairs and will be maintained using a randomized breeding strategy, ensuring that closely related animals are not bred together. The HS Breeding Core will ship HS rats (male and female) four or five times per year to Research Projects 1-3 over a four year period, with Research Project 1 receiving a limited number of rats in year 5. By providing a genetically diverse set of HS rats to Research Projects 1-3, this Core will provide the back-bone for the studies proposed in this P50, namely the genetic fine-mapping and identification of genes involved in behaviors related to substance use disorders.